Without limiting the scope of the invention, its background is described in connection with Radio Frequency Identification Systems (RFID), as an example. One such RFID system is disclosed in U.S. Pat. No. 5,053,774 by Schuermann et al. and assigned to Texas Instruments Incorporated. Another such system is described in U.S. patent application Ser. No. 07/981,635 by Meier et al., also assigned to Texas Instruments. These systems have interrogators that communicate with transponders. This communication is accomplished in a wireless fashion between an antenna of the interrogator and an antenna of the transponder. The interrogator RF module comprises transmit and receive circuitry for communicating with the transponder through the interrogator antenna. In order for the interrogator to transmit with maximum field strength, the interrogator antenna must be tuned to have a resonant frequency equal to the frequency input from the RF module. Several prior art methods for tuning the antenna to resonance are known.
One prior art method is to measure the power supply current into the supply voltage pin for the transmitter power stage of the RF module. The supply current into this pin will be at a maximum when the resonant frequency of the antenna is equal to the input frequency from the RF module.
Another prior art tuning method is to measure the field strength generated by the RF module into the antenna. In this method, a pickup coil is placed in proximity to the antenna. In the pickup coil an RF voltage is induced by the RF transmissions from the antenna. When the antenna is tuned to resonance, the induced voltage at the pickup coil will reach its maximum value. The difficulty with these prior art methods is the fact that to find the maximum current or measured field strength one must sweep the frequency of the antenna over a known frequency range then select the frequency at which the peak occurred.